“If our model about particle impacts is correct, then we would expect that most of the energy is converted into vibrations in the chip that propagate over long distances,” said UW-Madison graduate student Chris Wilen, the paper’s lead author.
To view the disruptions, researchers sent radio frequency signals into a four qubit system and, by measuring their excitation spectrum and performing spectroscopy on them, were able to see the qubits “flip” from one quantum state to another, observing that they all shift in energy at the same time, in response to changes in the charge environment. The significance of this research is that, given that sort of architecture, it puts some quantitative bounds on what you can expect in terms of performance for current device designs in the presence of environmental radiation.” “We even joked when we saw bad performance that maybe it’s because of cosmic rays. “We’ve always known this was possible and a potential effect - one of many that can influence the behavior of a qubit,” DuBois added. This alters the electric field as well as the thermal and vibrational environment around the qubits, resulting in errors, DuBois explained. These charged particles zoom through the materials in the device, scattering off atoms and producing high-energy vibrations and heat. When a particle impact occurs, it produces a wake of electrons in the device. Whereas bits are only susceptible to one type of error, under their temporary excited charge state, the delicate qubits are susceptible to two types of error, stemming from changes that can occur in the environment.Ĭharged impulses, even minute ones like those from cosmic rays absorbed by the system, can create a blast of (relatively) high-energy electrons that can heat up the quantum device’s substrate just long enough to disrupt the qubits and disturb their quantum states, the researchers found. For a few hundred microseconds, data in a qubit can be either a one or zero before being projected into a classical binary state. Unlike bits found in classical computers, which can exist only in binary states - zeroes or ones - the qubits that make up a quantum computer can exist in superpositions. Unless you can prevent that from happening you can’t perform error correction efficiently, and you’ll never be able to build a working system without that.” “Essentially, what this paper is showing is that if a high-energy cosmic ray hits the device somewhere, it has the potential to affect everything in the device at once. Correlated errors are very difficult to correct,” said co-author DuBois, who heads LLNL’s Quantum Coherent Device Physics (QCDP) Group. “For the most part, schemes designed to correct errors in quantum computers assume that the errors across qubits are uncorrelated - they’re random.
Additionally, the team linked tiny error-causing perturbations in the qubits’ charge state to the absorption of cosmic rays, a finding that already is impacting how quantum computers are designed. When a disruptive event occurs, such as a burst of energy coming from outside the system, it can affect every qubit in the vicinity of the event simultaneously, resulting in correlated errors that can span the entire system, the researchers found. In experiments performed at UW-Madison, the research team characterized a quantum testbed device, finding that fluctuations in the electrical charge of multiple quantum bits, or “qubits” - the basic unit of a quantum computer - can be highly correlated, as opposed to completely random and independent. Other co-authors included researchers at the University of Wisconsin-Madison, Fermi National Accelerator Laboratory, Google, Stanford University and international universities. This must be understood in order to build a functioning quantum system. In a new paper published in Nature and co-authored by LLNL physicist Jonathan DuBois, scientists examined quantum computing stability, particularly what causes errors and how quantum circuits react to them. Research by a Lawrence Livermore National Laboratory (LLNL) physicist and a host of collaborators is shedding new light on one of the major challenges to realizing the promise and potential of quantum computing - error correction.
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